1. Field of the Invention
This invention relates to a solid state imaging apparatus which employs a solid state imaging element such as a CCD (charge coupled device), and more particularly to a solid state imaging apparatus wherein a solid state imaging element itself is used as gain adjusting means.
2. Description of the Related Art
Solid state imaging apparatus which employ a solid state imaging element such as a CCD is generally constructed in such a manner as shown in FIG. 3.
Referring to FIG. 3, the solid state imaging apparatus is generally denoted at 10 and includes an optical system 12, an optical iris system 13 and a solid state imaging element 14 such as a CCD. An optical image of a subject 11 is formed by way of the optical system 12 and the iris system 13 on the CCD 14, at which it is converted into an electric signal, that is, an image signal.
Detailed construction of the CCD 14 is shown in FIG. 4. Referring to FIG. 4, the CCD shown is of the frame interline transfer type and includes a frame accumulating section 20 and a horizontal transfer section 21.
The accumulating section 20 includes a plurality of vertical transfer sections (vertical shift registers) 23 formed corresponding to individual columns of picture elements 22 which are disposed in a two-dimensional arrangement, that is, in rows and columns. Accumulated charge is transferred line by line in the horizontal transfer section 21 and converted into a signal (voltage signal) by a voltage converting section 24. An image signal after conversion into a voltage is outputted to a terminal 26 by way of a horizontal output amplifier 25. Referring back to FIG. 3, an image signal obtained from the CCD 14 is supplied to a signal processing system 15, from which it is outputted as a video signal (composite signal) of the NTSC system.
The image signal from the CCD 14 is supplied also to a detecting circuit 16, at which an amplitude level of the image signal is detected. A detection output of the detecting circuit 16 is supplied to a motor driver 17 so that an iris driving motor 18 is controlled in accordance with the thus received detection output to effect iris control so that the image signal may present a predetermined level.
While such iris control is popular exposure controlling means for the adjustment of the level of an image signal in this manner in a conventional solid state imaging apparatus, according to the iris control, when the subject 11 to be photographed is particularly bright, the iris 13 will be controlled until the aperture of the lens or optical system 12 is reduced to its minimum aperture.
However, if the iris 13 is controlled to the minimum aperture position in this manner, then small aperture fading is caused by diffraction of light at the iris 13, and this makes a cause of deterioration of the picture quality. However, if the minimum aperture is adjusted so that small aperture fading may not take place, then sufficient iris control cannot be effected now and an image signal cannot be restricted to a necessary level.